On the work of adhesion and peel strength between pressure sensitive adhesives and the polymeric films used in LCD devices

Peel strength, a convenient measure of bond strength in adhesive/adherend systems, is known to be a function of various factors such as the thermodynamic work of adhesion, rate of measurement, thermal history, and temperature. Generally, it is believed that the work of adhesion is primarily involved in the first stage of adhesion through wetting phenomenon and beyond that its role diminishes in that the portion of thermodynamic contribution to actual bond strength is insignificant. In practice, however, we often observe that a suitable surface treatment increases the surface energy of the substrate, which further enhances the bond strength. One practical example is the surface treatment carried out in LCD industry to obtain sufficient bond strength between pressure sensitive adhesives and polymeric films. To further our understanding of the effect of surface treatment, we attempted to establish a possible correlation, if any, between the thermodynamic work of adhesion and peel strength. For this, we carefully measured the contact angles of water and diiodomethane against various polymeric films, and calculated the surface energy and the thermodynamic work of adhesion using the two widely used approaches: Young-Fowkes-Girifalco-Good, and Wu methods. Before establishing a correlation, some general aspects of the above two methods are discussed. The values of the work of adhesion obtained were compared with the measured peel strength. Indeed, we observed a clear correlation between the two quantities: the increase of the work of adhesion led to the increase of peel strength. As a reason for this correlation, we proposed that the increase of surface energy might be associated with the increase of various surface functional groups, which, in turn, contributed to the formation of chemical bonding with the PSA leading to the increase of peel strength.     

旋翼无人机在移动平台降落的控制参数自学习调节方法

无人机设备能够适应复杂地形,但由于电池容量等原因,无人机无法长时间执行任务。无人机与其他无人系统（无人车、无人船等）协同能够有效提升无人机的工作时间,完成既定任务,当无人机完成任务后,将无人机迅速稳定地降落至移动平台上是一项必要且具有挑战性的工作。针对降落问题,文中提出了基于矫正纠偏COACH(corrective advice communicated humans)方法的深度强化学习比例积分微分(proportional-integral-derivative, PID)方法,为无人机降落至移动平台提供了最优路径。首先在仿真环境中使用矫正纠偏框架对强化学习模型进行训练,然后在仿真环境和真实环境中,使用训练后的模型输出控制参数,最后利用输出参数获得无人机位置控制量。仿真结果和真实无人机实验表明,基于矫正纠偏COACH方法的深度强化学习PID方法优于传统控制方法,且能稳定完成在移动平台上的降落任务。     

Adhesion Performance and Microscope Morphology of UV-Curable Semi-interpenetrated Dicing Acrylic PSAs in Si-Wafer Manufacture Process for MCP

UV-curable acrylic pressure-sensitive adhesives (acrylic PSAs) have many applications in industry. As the Si-wafers become thinner, the acrylic PSAs for MCP need to show proper adhesion and leave little residue on the Si-wafer after UV irradiation when released from the dicing tapes. Strong adhesion is required in the dicing process to hold the Si-wafer before UV irradiation. On the other hand, weak adhesion strength is required after UV irradiation to prevent damage to the Si-wafers during the pick-up process. This study employed semi-interpenetrating polymer network-structured dicing of acrylic PSAs in the Si-wafer manufacture process. The binder PSAs contained 2-ethylhexyl acrylate (2-EHA) and acrylic acid (AA). The adhesion performance of the peel strength on a Si-wafer was examined as a function of the UV dose. The results showed that the abovementioned two requirements were achieved using semi-IPN dicing acrylic PSAs using a hexafunctional acrylate monomer and a UV-curing system. FE-SEM and XPS revealed little residue on the wafer after removing the tape. This paper suggests the optimal conditions for the curing agent, the additional hexafunctional monomer, photoinitiator and the coating thickness.     

UV-Ozone Surface Treatment of SBS Rubbers Containing Fillers: Influence of the Filler Nature on the Extent of Surface Modification and Adhesion

SBS rubbers containing different loadings of calcium carbonate and/or silica fillers were surface treated with UV-ozone to improve their adhesion to polyurethane adhesive. The surface modifications produced on the treated filled SBS rubbers have been analyzed by contact angle measurements, ATR-IR spectroscopy, XPS and SEM. The adhesion properties have been evaluated by T-peel strength tests on treated filled SBS rubber/polyurethane adhesive/leather joints. The UV-ozone treatment improved the wettability of all rubber surfaces, and chemical (oxidation) and morphological modifications (roughness, ablation, surface melting) were produced. The increase in the time of UV-ozone treatment to 30 min led to surface cleaning (removal of silicon-based moieties) due to ablation and/or melting of rubber layers and also incorporation of more oxidized moieties was produced. Although chemical modifications were produced earlier in an unfilled rubber for short time of treatment with UV-ozone, they were more noticeable in filled rubbers for extended length of treatment, mainly for S6S and S6T rubbers containing silica filler. The oxidation process seemed to be inhibited for S6C and S6T rubbers (containing calcium carbonate filler). On the other hand, the S6S rubber containing silica filler and the lowest filler loading showed the higher extent of modification as a consequence of the UV-ozone treatment. The UV-ozone increased the joint strength in all joints, more noticeably in the rubbers containing silica filler, in agreement with the greater extents of chemical and morphological modifications produced by the treatment in these rubbers. Finally, the nature and content of fillers determined the extent of surface modification and adhesion of SBS rubber treated with UV-ozone.     

Improvement of adhesion between poly(ethylene terephthalate) and polyethylene

Three-layer films were prepared with polyethylene (PE) and poly(ethylene terephthalate) (PET) films as the outer layers and a film of high-density polyethylene (HDPE)/ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) terpolymer blend as the inner layer using compression molding. E-MA-GMA, an elastomer containing an epoxy functional group, was used as the adhesion promoting agent in the multilayer films. The effects of processing temperature, pressure application time and elastomer concentration on adhesion were investigated. The adhesion strength between PE and PET films increased with increasing bonding temperature, bonding time and elastomer concentration. From contact angle measurements, it was observed that the work of adhesion between the polymers increased with increasing amount of elastomer. Improved adhesion between PET and HDPE with 30% elastomer films was confirmed by SEM analyses of the film layers. Using FT-IR analysis of PE/HDPE-30% elastomer/PET delaminated film, the decrease in peak intensity of the epoxy groups tends to indicate reaction of epoxy functionality with functional groups in PET.     

Shear and peel stress analysis of an adhesively bonded scarf joint

The shear and peel stress distributions in a scarf joint made of two isotropic adherends with blunt adherend tips are analysed using a linear elastic analysis. The limits of the analysis with respect to adherend tip thickness have been investigated. A finite difference method is used to solve the differential equations for the shear and peel stress distributions over the joint. The boundary conditions used limit the analysis to the two adherends having the same thicknesses, lengths, and material properties. The adherends are modelled as plates with extensional and bending stiffnesses bonded together with an elastic interlayer. The stresses across the adhesive layer are assumed to be constant. The current analysis applied to cases known from the literature shows good agreement with the shear stresses but the peel stresses are overestimated.     

Effects of blending and coating methods on the performance of SIS (styrene-isoprene-styrene)-based pressure-sensitive adhesives

SIS (styrene-isoprene-styrene)-based pressure sensitive adhesives (PSAs) were prepared by melt- or solution-blending. In the coating process, two methods were used: solution coating and melt coating. The performances of the PSAs were found to be different, depending on which of these two blending or coating methods was used. In this study, we investigated the relationship between the viscoelastic properties and the performances of the SIS-based PSAs using different blending and coating methods. Three methods were used: (1) melt-blending and melt-coating, (2) melt-blending and solution-coating and (3) solution-blending and solution-coating. PSAs applied using melt-blending/melt-coating (M—M) have higher peel strength and probe tack than PSAs applied using melt-blending/solution-coating (M—S) and solution blending/solution coating (S—S). However, PSAs applied using M—M blends have lower holding power and SAFT (Shear adhesion failure temperature) than PSAs applied using M—S and S—S blends. The viscoelastic properties and GPC curves of M—S blends were similar to M-M blends, while the peel strength and tack of M—S blends were similar to S—S blends. Therefore, it was concluded that the blending process had more effect on the viscoelastic properties and shear creep of PSAs than the coating process.     

Studies on the adhesion behavior of water-based adhesives blended with asan gum

The influence of asan gum, a locally available waste material obtained from the Terminalia alata tree, in blends with waterborne natural rubber adhesive and poly(vinyl acetate), on the lap shear strength and the peel strength has been investigated. Both the strength values increase, even with a small quantity of the gum. At a higher gum content, both these parameters, however, decrease. Fourier Transform Infrared studies reveal that there is no covalent bonding between the gum and the adhesives, although some hydrogen bonding exists in the poly(vinyl acetate) blend. Morphological studies reveal mechanical interlocking of the adhesive in the substrates. The pseudoplastic nature of the gum-modified waterborne adhesives has been confirmed from rheological studies using a Brookfield viscometer. The higher lap shear and peel strength values of the gum-modified adhesives compared to the control adhesives are attributed to the higher shear modulus of the former. The 100% modulus and tensile strength of the adhesives blended with the gum are also higher, compared to their controls.     

Studies of Terminalia bellerica (Bahera), a natural gum,as an additive in a water-based adhesive composition

Terminalia bellerica (Bahera), a natural gum, was used as an additive in water-based natural rubber adhesive formulations. The lap shear strength of joints with wood substrates increased with the increase of natural gum concentration up to 80 phr (parts per hundred parts of rubber) loading. Similarly, the peel strength of the canvas to canvas joints increased with the incorporation of the gum. The results were explained with the help of viscoelastic properties, morphology and surface chemistry of the rubber–gum mixtures.     

Stress Analysis of Tubular Adhesive Joints with Delaminated Adherend

The use of adhesive joints is becoming increasingly important in aerospace, automotive and other industries where the use of traditional fasteners is discouraged. When using composite adherends, the use of adhesively bonded joints is preferable rather than the traditional bolts and other types of fasteners, because they do not require holes, thereby removing the problems of stress concentrations around the holes. However, when using an adhesively bonded joint, there will be concentrations of the distributions of shear and peel stresses within the adhesive layer which should be controlled effectively. Therefore, the investigation of such stress variation has attracted many researchers. The aforementioned stress distributions become more complicated if the composite adherend contains a pre-existing delamination. Delamination is one of the most common failure modes in laminated composite materials; it can occur due to sudden impact by an external object, during the manufacturing process (e.g., during the filament winding process), or as a result of excessive stresses due to an applied load. It is clear that the existence of a delamination in any composite structure causes a reduction in its stiffness and in some critical situations, it may cause complete failure. This paper investigates the effect of delamination on the structural response of an adhesively bonded tubular joint with composite and aluminum adherends. The finite element method, using the commercial package ABAQUS, is used to conduct a parametric investigation. The effects of the delamination's spatial location, length, width, and the applied loading are studied. Results provide interesting insight (not necessarily intuitive) into the effect of an interlayer delamination on the stress distribution within the adhesive.